Puree the tomatoes in a blender until smooth. Strain puree through a medium-mesh sieve, pressing with a rubber spatula to get out all of the juices. Discard any solids. Set tomato puree aside.
In a non-reactive saucepan heat the oil over medium-high heat. Add the onion and cook until softened, 3 to 4 minutes; do not allow to color. Add the tomato puree and all remaining ingredients except the vinegar and salt and bring to a boil. Reduce heat to a simmer and cook, stirring occasionally at the beginning and more often towards the end of the cooking time, for about 1 hour or until thickened. Add the cider vinegar and salt and cook for 25 to 30 minutes more, until the consistency of ketchup. Adjust seasoning with salt if needed. Set aside to cool. Store in non-reactive airtight containers in the refrigerator for up to 1 week.
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